Silicon controlled rectifiers (SCRs) are used extensively for controlled (regulated) AC-DC power supply applications. Where current flow in the circuit is to be controllably interrupted, a (PNPN) gate turn-off (GTO) type SCR is often used. One such application, in which a GTO SCR is used to controllably gate or switch the current flow path between a diode-rectified AC input source and a storage capacitor in accordance with whether or not the line voltage exceeds the capacitor voltage, is described in U.S. Pat. No. 4,675,798, to S. R. Jost et al, entitled "Direct Coupled Switching Power Supply With GTO SCR Switching Element", assigned to the assignee of the present application and the disclosure of which is incorporated herein.
In such circuits, in order to turn off the GTO-SCR device, and thereby interrupt the flow of current in the circuit path to which the anode and cathode of the SCR are coupled, it has been customary practice to controllably remove current from its cathode gate. For a detailed discussion of GTO-SCR operation, attention may be directed to various texts on the subject, such as that entitled "Semiconductor Power Devices", by S. Ghandi, Chapter 5, pp. 190-261, published by Wiley & Sons.
As discussed in an article entitled One-Dimensional Analysis of Turnoff Phenomena for a Gate Turnoff Thyristor", IEEE Transactions on Electron Devices, Volume ED-26, No. 3, Mar. 1979, pp. 226-231, the turn-off sequence of a GTO-SCR may be characterized by three successive periods of time -- storage time, fall time and tail time. The current which flows during the latter period (tail current) is a result of PNP transistor action (for cathode gate turn-off), when charge (electrons), stored in the base of the PNP transistor portion of the thyristor, is forced by the collector-base junction capacitance displacement current across the base-emitter junction. Tail current can cause device failures if its magnitude is large enough to cause substantial power dissipation, which tends to be localized at `hot spots`, leading to melting of the device, as explained in an article (source unidentifiable) entitled "The Current Status of the Power Gate Turn-Off Switch GTO", by M. Okamura et al, pp. 39-49, circa 1977-1978.
Unfortunately, proposed techniques to reduce tail current have typically involved an increase in processing complexity in the design and manufacture of the SCR, specifically by incorporating into the SCR device structure some mechanism for decreasing the PNP transistor current gain, for example by the introduction of carrier killers (via gold doping, irradiation), or the use of anode shorts (i.e. built-in resistors or diodes in parallel with the emitter-base junction). Moreover, because such modifications of the structure of the SCR bias its operation in favor of turn-off, they tend to negatively impact on-state characteristics of the device.